334 GEOLOGICAL SLTiYEY OF THE TEEEITOEIES. 



Portions of a second individual of this species, or of L. proriger, were 

 found on the Fox Canon. They belonged to a larger animal, one equal 

 to the 2sew Mexican first described. Professor Mudge has fragments of 

 still larger specimens. 



The principal specimen above described was excavated from a chalk 

 bluff. Fragments of the jaws were seen lying on the slope and other 

 13ortions entered the shale. On being followed, a part of the cranium 

 was taken from beneath the roots of a bush, and the vertebrge and limb- 

 bones were found further in. The vertebral series extended parallel 

 with the outcrop of the beds, and finally turned into the hill, and was 

 followed so far as time would permit. It was abandoned at the auterior 

 caudal vertebrae for more favorable circumstances or a more persevering 

 excavator. 



The outcrop of the stratum was light yellow. The concealed part of 

 the bed was bluish. Yellow chalk "left on the specimens in thin lay- 

 ers became a white or nearly so. The yellow and blue strata are defi- 

 nitely related in most localities, the former being the superior, but in 

 others they passed into each other on the same horizon. 



TESTUDFN^ATA. 

 PEOTOSTEGA, Cope. 



Proc. Amer. Phil. Soc, 1871, p. 173 ; loc. cit, March, 1872. 



This genus is the type of a new family of tortoises of the suborder 

 Athec(c, characterized by the lack of expansion of the ribs into 

 a bony roof or carapace, and the development of dermal bones only 

 on the upper surfaces. The dermal bones consist of large plates lying 

 above the ribs, which have no sutural union with each other ; of small 

 vertebriil shields on the dorsal line, and of thin, marginal bones, which 

 have no sutural union with each other or with the other bones. The 

 vertebrie preserved possess ball-and-socket joints, and have flat neural 

 arches, with widely spreading articular processes. The humeri are flat, 

 and furnished with an enormous deltoid crest. The fore limbs were 

 very long, and formed flippers like those of the marine turtles of the 

 present seas. The bones of the head were very light and thiu. and 

 mostly united by squamosal or overlapping sutures. The mandible pre- 

 sented tlie elements usual in the marine turtles, and had no angle. It 

 exhibits a deep pterygoid fossa, and is very light. The constitution of 

 the bones is rather dense, and there are no medullary cavities whatever. 

 The superficial layer is very thin and striate. The bones are all very 

 fragile. The fore limb discovered several years since in the Cretaceous 

 of Mississippi, near to Columbus, with vertebra? andteeth, of Platecarpiis 

 tym2)a)uficus, which was referred by Dr. Leidy to that species, probably 

 belongs to Frotostega. It represents a species distinct from the P. gigas^ 

 which may be called Frotostega tuberosa, Cope, and diflers from P. gigas 

 in the more elongate form of the humerus, with sui^erior position and 

 more enlarged form of the bicipital process. The large deltoid crest 

 apj)ears to be also much more prolonged. A third species, or allied 

 genus, has also been discovered in the green-sand of Xew Jersey. It is 

 represented by a fragment of a gigantic humerus, which was rightly 

 regarded as pertaining to a turtle; though he never described it. Dr. 

 Leidy figured it,* and referred '-to the gigantic Mosasaurus.^i I refer 

 it i^rovisioually to F.otostega with the name P. neptunia. The humerus 



* Cretaceous Reptiles of North America. Tab. VII, Eig. 4. t Loc. cit., p. 43. 



